Although your speed isn't great, it sounds pretty much normal, though it does depend what area you live in. Ofcom have been criticising companies like BT recently for their "up to..." type speed advertising. Here's an article about ADSL speed degradation from BBC news:
http://news.bbc.co.uk/1/hi/magazine/8171984.st m
If you have neighbours on the same service, or friends living very close by that have their broadband through the phoneline, ask them what sort of speeds they get, and see if they're comparable. As stated, it may just be your distance from the exchange.
Before I moved house earlier this year, I had BT Broadband and was getting 6Mb roughly. When I moved here I was told the max I could get was 1Mb, even though I live in a town centre in the Midlands, simply because I'm at "the end of the line" even though I'm physically close to the exchange. I ended up having to ditch BT and get fibre optic cable installed. I now get 9Mb + pretty much constantly.